The present invention relates generally to fiberoptic devices and, more specifically, to fiberoptic devices with long focal length GRIN (gradient-index or graded refractive index) fiber lens.
A GRIN lens has a refractive index whose value varies with radial distance from the axis of the lens. The nontrivial variation in refractive index causes light refraction and gives the GRIN lens focusing capabilities. See, e.g., U.S. Pat. No. 6,542,665, the entire disclosure of which is incorporated herein by reference. Many optical devices use lenses to focus, collimate, or expand light beams. When a GRIN fiber is cut into a specific length, the short length GRIN fiber can focus a light beam. For example, when a GRIN lens is fused to a terminal end of an optical fiber, it can focus the light beam emitted by the optical fiber. See, e.g., William A. Reed et al., “Gradient-index fiber-optic microprobes for minimally invasive in vivo low-coherence interferometry,” Optics Letters, Vol. 27, No. 20, Oct. 15, 2002, the entire disclosure of which is incorporated herein by reference. However, the focus length of the GRIN lens is very short (typically around 0.35 mm), which seriously restricts its applications.